As the specifications on materials and the steps in the processing of them become more stringent; and with the expanding of the applications where the materials are to be used, ever greater constraints are being encountered. The major continuous processing technique used in the art is the performing of an operation at a station on a quantity of a material. The material itself may be the web; as for examples a film or a layer of dielectric supporting material on which in the future there is to be the mounting of electronic components, or the fabrication of structural members. The material may be a finely divided particulate supported by a web.
One of the operations performed in the processing at a station is the application of heat in order to alter one or several properties of the material being processed. In the recent timeframe in the application of heat, the specifications that have to be met, have become more complex involving more than one type of alteration of the material. A particular example is the formation of some types of dielectric sheet materials into intermediate manufacturing products. In these types of operations, a coarse reinforcing material is coated or impregnated with a resin that in turn is suspended in a solvent or a liquid vehicle. With this type of material to be processed, the heating operation at a processing station includes the physical alteration of properties in drying and a precise portion of a chemical reaction in partial curing. The physical alteration of drying takes place by evaporation and by diffusion through the material both at independent rates. In the chemical alteration there should be a limit to the chemical reaction so that it only goes so far and is stopped even if the reaction is exothermic. The intermediate manufacturing product is known in the art as "prepreg" or "B stage" material. It is a stable material that is typically in the form of a sheet with the solvent removed. The chemical reaction of curing is only partially complete such that at elevated temperatures consolidation and fusing is possible. Further deformation, such as will occur in lamination or consolidation then takes place at a final assembly and full curing operation.
Accompanying the considerations in achieving the meeting of specifications, environmental concerns are becoming of increasing importance. Attention is being given to energy consumption and to the collection of volatile products driven off at processing stations. In the above example of "B stage" material, in the art, large vertical structures are used at substantial cost in providing an energy retaining and atmospherically enclosed environment for the process steps.
Efforts have been underway in the art to gain the benefits of energy efficiency and depth of penetration of microwave energy in web type processing systems.
In U.S. Pat. No. 4,234,775 the drying of a web of material is accomplished using a serpentine wave guide that goes back and forth across the web while hot spots are controlled by preventing the formation of a standing wave in the wave guide.
In U.S. Pat. No. 4,402,778 a laminating process line is described wherein laminations are pressed together into a web and in the process line the laminations are partially cured in a field between a pair of flat plates with final curing taking place in a subsequent station. This type of approach requires that the energy be in the radio frequency (RF) range and that heavily absorbing materials already in the "B stage" be used.
In PCT International Publication WO91/03140 of PCT Application PCT/AU90/00353, the drying of surface coatings is performed through the use of a microwave applicator that has independent sections above and below a web with each section having an antenna that extends length of the section.
A need is present in the art for greater precision in temperature and environmental control in the application of microwave technology to material processing.